Process for the formation of electric insulating coating on electric iron sheet



Nov. 11. 1969 TOSHM mm ET AL 3,477,881 rnocnss FOR THE FORMATION OF ELECTRIC INSULATING comma on ELECTRIC IRON sunn'r Filed April 16. 1968 Amount of Cr dissolbed I l l i I 5' l5 so minute Period of time of sheaf being placed in furnace v l l I l l l 0 IO 20 3O 4O 5O 60 second Period of time of sheaf being placed in furnace 1N VE /V 7' 0/? Tosh/ya Wade Kanao Akanuma Tosh/hike Takafa United States Patent Int. (31. 623i 7/26 US. Cl. 1486.2 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a process for forming an electric insulating coating on an electric iron sheet produced from silicon sheet or usual steel, comprising applying on the surface of the electric iron sheet a reducing solution containing hexavalent chromium ion, at least one bivalent metal ion, said solution having been added with at least a member selected from water-soluble saturated dicarboxyl acids having at least 4 carbon atoms and/or further added with phosphoric acid and/or boric acid and heating it for 10 to 180 hours at 150 to 500 C.

This application is a continuation-in-part of application Ser. No. 434,408 filed Feb. 23, 1965, now abandoned.

This invention relates to a process for the formation of electric insulating coatings on electric iron sheets which are used for transformers, electric generators, electric motors and the like.

In particular, the invention relates to a process for forming easily and with a low cost on an electric iron sheet an electric insulating coating having improved interlayer resistance, heat resistance, space factor, workability, and weldability by adding in an aqueous solution containing hexavalent ions and alkaline earth metal ions a re ducing and water-soluble organic compound having miscibility of above minutes with said aqueous solution at room temperature, applying the solution on the surface of an electric iron sheet produced from silicon steel or usual steel, and heating the thus coated surface.

As the processes for forming electric insulating coatings on electric iron sheets produced from silicon steel or usual steel, there have been known, for example, a process wherein an oxide coating of ferrosoferric oxide, etc., is formed on an iron sheet When the sheet is formed by hot rolling silicon steel as the electric insulating coating, a process wherein a varnish is applied on the surface of a rolled silicon steel sheet, and a process wherein a coating of a phosphate is formed on the surface of a silicon steel sheet.

However, since such a steel sheet has hitherto been subjected to annealing at about 800 C. in order to relieve strains after working, such as, shearing and punching for improving the qualities of the electric iron sheets, the surface coating by the varnish or ferrosoferric oxide is stripped or carbonized by the annealing and the phosphate coating is extremely reduced in the electric. insulating property after annealing. Therefore, there have been proposed a process wherein fine particles of muscovite are suspended in a phosphate solution and the suspension is applied on the surface of a silicon steel sheet followed by heating, and a process wherein lime, magnesia, etc., is coated on the surface of a silicon steel sheet and the sheet is annealed at a high temperature to form an electric insulating coating. However, there are drawbacks in the 3,477,881 Patented Nov. 11, 1969 former process that specific care are needed in handling and equipment for the solution caused by adopting the dense acid, the hygroscopic property of the coating is large if printing is insufliciently applied, which reduces the electric insulating property of coating, and the use of such a dense phosphoric acid is uneconomical, and in the latter process there are defects that the electric iron sheet is liable to be influenced by the high-temperature treatment and if the iron sheet is overheated, the electric iron sheet is deformed and the magnetic property of the iron sheet is deteriorated. Moreover, there are other defects in the latter process that coating itself is rough and lusterless, which result in damaging easily the coating.

As a process for giving electric insulating coating on an electric iron sheet, a phosphate-coating treatment has been generally adopted, but as mentioned above, in such a phosphate-coating treatment the interlayer resistance may be extremely reduced by strain-relieving annealing at about 800 C. and there is trouble in weldability. That is, if there is a phosphate film on the surfaces of an electric iron sheet, phosphorus is easily entered in welded portions at welding the laminated electric iron sheets to form a core, which results in reducing the strength of the welded portions, then in order to improve weldability, the content of phosphorus in coating must be reduced.

Therefore, an object of the present invention is to form on the surface of an electric iron sheet coating having, in particular, improved workability and adhesive property whereby prolonging the life of punching and shearing tools as well as occurrence of stripping and the reduction of the electric insulating property at punching and shearing are prevented.

Another object of this invention is to provide a highquality electric iron sheet by adding a specific reducing water-soluble organic compound in a coating solution, applying the coating solution on the surface of a silicon steel sheet and heating the coating at a comparatively low temperature to form a green, transparent, and lustrous coating containing a chromium compound and having improved scratching resistance without reducing the magnetic property of the steel sheet.

An additional object of this invention is to provide an electric iron sheet showing improved electric insulating property and weldability even after subjecting to strainrelieving annealing in a reducing atmosphere at high temperatures of about 800 C.

In the accompanying drawing,

FIG. 1 is a graph showing the relation between the period of time for which an iron sheet coated with a coating solution is placed in a furnace (at the furnace temperature of 500 C.) and the amount of Cr+ dissolved from the coating layer (of 30 mg./ sq. cm.) during the immersion of the coating in boiling water for 3 minutes, and

FIG. 2 is a graph showing the relation between the period of time for which an iron sheet coated with a coating solution is placed in a furnace (at the furnace temperature of 500 C.) and the temperature of the sheet.

The process of this invention comprises the step of applying on the surface of an electric iron sheet an aqueous solution prepared by adding to an aqueous solution containing 0.520% of hexavalent chromium ions and 03-23% of at least one bivalent metal ion selected from the group consisting of alkaline earth metal ions, copper ion, cadmium ion, and barium ion, at least one kind of water-soluble saturated dicarboxylic acids having miscibility of above 15 minutes with the aqueous solution at room temperature, and the step of heating it for 10-180 seconds in a temperature range of to 300 C.

Further, according to the process of this invention, by

adding 01-20% of phosphoric acid, calcium phosphate or an oxyacid of phosphorus such as. hypophosphoric acid, the temperature range for heating can be enlarged up to 500 C. while obtaining easily the uniform surface at coating and after baking. However, the addition of an excess amount of the phosphorus compound reduces the pH of the solution and lowers the preservative stability.

When a copolymer of polyethylene glycol, polyvinyl methyl ether, and maleic acid is incorporated in the solution without the presence of the phosphorus compound as mentioned above, the suitable heating conditions are ZOO-280 C. and 15-30 seconds and by the heating conditions, a coating having good appearance as well as improved workability, electric insulating property and weldability.

Into the above-mentioned electric insulating coating composition may be further added 0.010.5% of a nonionic surface active agent of polyethylene glycol series to improve wetting property whereby the surface of an electric insulating iron sheet is coated uniformly with the coating composition even though the surface of the iron sheet is stained in some extent.

In particular, by the addition of the nonionic surface active agent of polyethylene glycol series, coating having a uniform thickness can be formed on the electric iron sheet and the coating work is also improved. Further, by the addition of 25% of boric acid, the luster of the surface can be effectively improved.

As the compounds giving hexavalent chromium ions are adopted chromium trioxide, chromic acid, Watersoluble chromates, water-soluble bichromates, etc., and as the compound giving bivalent metal ions, for instance, alkaline earth metal ions, are preferable alkaline earth metal oxides, such, CaO, MgO, SrO, BeO, BaO, etc.

In this case, the addition of these alkaline earth metals in an amount of 1-5% remarkably improves the electric insulating property after annealing. However, also bivalent metal ions such as Cu ion, Zn ion, Cd ion and the like may be used instead of alkaline earth metal ions. If desired, by adding only an alkaline earth metal salt of water-soluble chromic acid or bichromic acid, hexavalent chromium ions may be added simultaneously with one or more alkaline earth metal ions, copper ion, and cadmium ion.

The water-soluble organic compound having miscibility of above 15 minutes with the aqueous solution of such a chromium compound at room temperature is, in other words, a reducing compound which does not cause the reaction instantly with an aqueous solution of, for example, chromium trioxide or an alkaline earth metal salt thereof at room temperature, but causes the reaction on the surface of a steel sheet at an elevated temperature, that is, water-soluble dicarboxylic acids having 4 or more than 4 carbons and having the boiling point, sublimation point and decomposition point of more than 150 C. At least one kind selected from these saturated dicarboxylic acids should be added.

Miscibility of above 15 minutes at room temperature is defined by the experiences in coating works. As the coating solution for carrying out coating easily and with a low cost in this invention, an aqueous solution is preferable. Among organic compounds having solubility in water and a reducing property, methyl alcohol, ethyl alcohol, formalin, formic acid, acetic acid and the like having low boiling points are unsuitable in the process of this invention since they evaporate before the filmforming reaction occurs. Further, such compounds that cause instantly the reaction with the aqueous solution of, for example, chromium trioxide or an alkaline earth metal salt thereof, such as, hydroquinone and pyrogallol do not contribute to the formation of coating even though they have high boiling points and reducing properties.

The inventors have found that as the compounds satisfying the above-mentioned factors in the process of this ing 4 or more than 4' carbon atoms such as succinic acid,

glutaric acid, adipic acid, pimelic acid are most suitable in the adhesion property, Working workability and punchability. In particular, succinic acid and adipic acid are preferable from the economic point of view.

As to hexavalent chromium ion, if the content of it is lower than 0.5%, the interlayer resistance and heat resistance of coating are low. While, if the content of hexavalent chromium ion is above 20% coating coarsened and space factor of coating is reduced. Furthermore, if the content of the organic compound is less than 0.1%, the film-forming elfect of the organic compound is not effected and if the content is above 8%, the film-forming effect is not increased more as well as the heat resistance of coating becomes insufficient.

As to bivalent metal ions such as alkaline earth metal ion, copper ion, or cadmium ion, if the content of it is less than 0.3%, it does not contribute to the improvements in stability of the coating solution and interlayer resistance and heat resistance of coating. However, even though the metal ion is added in the aqueous solution in an excess proportion, it causes only increase in the deposition of the metal oxide in the system roughening of the appearance and reduction of the space factor. The preferable upper limit for the addition of the metal ion is up to 8%.

The above-mentioned coating solution is applied uniformly on the surface of an iron sheet as a thin layer and heated for 10-180 seconds at the surface temperature of the iron sheet of ISO-300 C. However, if the heating conducted at conditions out of the above ranges of period and temperature, coating is not formed or decomposed.

Therefore, according to a preferred embodiment of the process of this invention, an aqueous solution as a basis solution containing 1-40% of chromium trioxide and 0.1-8% of one or more water-soluble saturated dicarboxylic acids having at least 4 carbon atoms such as succinic acid, glutaric acid, adipic acid and pimelic acid is added with above 1%, preferably 125%, of one or more metal oxides, such as, CuO, ZnO and CdO or alkaline earth metal oxides, such as, MgO, CaO, BeO, SrO, BaO, etc., applied uniformly and thinly on the surface of an electric iron sheet, and thus formed coating is heated for 10-180 seconds at the surface temperature of iron sheet of -300" C. to give uniform coating on the surface of the iron sheet.

EXAMPLE 1 Adipic acid Phosphoric acid 8 Water cc 200 An aqueous solution consisting of the above materials was applied uniformly and thinly on the surface of an electric iron sheet and the coating was heated to 250 C. of the surface temperature of the electric iron sheet to give uniform coating of 20-30 mg./sq. cm. in thickness. The coating had extremely good adhesive property and when the iron sheet was bent along a rod of 10 cm. in diameter to the angle of 180 C., no stripping occurred in the coating. The space factor of the thus coated electric iron sheet was 9899% The solution of the present invention has also an excellent stability. No deposit was perceived after having left it for more than one month at a temperature of 500 C. However, when using ethylene glycol or glycerine instead of adipic acid a green-colored deposit was produced.

As mentioned above, beautiful coating is obtained by using the before mentioned coating solution but by further adding boric acid in the coating solution, the luster of coating can be further increased. The addition amount of boric acid is preferably below 8% since if it is above 8%, it is difficult to maintain the boric acid in the dissolved state in the aqueous solution.

In the above mentioned embodiment of the process of this invention, the temperature range of the iron sheet for forming coating is narrow but the upper limit of the heating condition can be extended to about 500 C. by adding a small amount of the phosphorus compound. In fact, if the coating containing no phosphorus compounds is heated to above 300 C. the organic components in the surface coating of the electric iron sheet are decomposed but the decomposition can be prevented by the addition of phosphoric acid, calcium dihydrogen phosphate, hypophosphoric acid, etc. Among them, calcium dihydrogen phosphate is comparatively inexpensive and very effective for increasing heat resistance. However, even in this case, if the coating is heated to above 500 C., the objective coating can not be formed. Of course, even though a small amount of phosphoric acid is added in an aqueous solution containing only chromium trioxide and the metal oxide and containing no organic compounds as mentioned above, the coating formed by heating the coating at about 200 C. in the surface temperature of the iron sheet is water-soluble. That is, the addition of phosphoric acid is effective only in the case where the abovementioned organic compound is contained in the aqueous solution or coating solution. The content of phosphoric acid is, however, preferably below since the addition of 520% of phosphoric acid has no more effects for enlarging the temperature range of iron sheet, though a good film is produced thereby, and on the contrary, reduces heat resistance and weldability of coating.

If the surface of an iron sheet is clear and the coating solution can be applied uniformly, there are no problems, but if wetting property for coating is bad, the addition of a surface active agent is preferable. The addition amount of such a surface active agent is preferably below 0.5%, that is, even if it is added in an amount more than 0.5%, no more improvement in the wetting property is not obtained or, on the contrary, foaming may be formed, which reduces the appearance of coating. As the surface active agent for the purpose in the process of this invention, nonionic surface active agents of polyethylene glycol series are suitable.

The following is other embodiment of the process of this invention wherein the above-mentioned additional materials are incorporated.

EXAMPLE 2 Chromium trioxide g Magnesium oxide ..g 2 Adipic acid g 4 Boric acid g 1 Surface active agent (polyethylene glycol lauryl ether) g 0.1 Water cc 100 An aqueous solution consisting of the above materials was applied uniformly and thinly on the surfaces of an iron sheets and the sheets were heated in a furnace of 500 C. for various periods of time to form coatings. The iron sheets were immersed in boiling water for 5 minutes and the proportions of dissolved Cr+ were analyzed quantitatively, the results of which are shown in FIG. 1 and the temperatures of the iron sheets in this case are shown in FIG. 2. As is clear from FIG. 1, there is a range wherein the temperature range for the surface temperature of iron sheet is narrow but the proportion of dissolved Cr+ is extremely low. This is considered to be attributed to the formation of stable compounds by the reaction of ions (HCrO Cr O- etc.) formed by the resolution of chromium trioxide with adipic acid and Mg+ on the surface of the iron sheet. Hence, the advantage of this invention is in a point that the compounds cause reaction at a comparatively low temperature to form stable coating.

In Example 2, extremely stable compounds are formed when the system is heated in a temperature range of -270 C. in sheet temperature, but if the sheet temperature is further increased, the proportion of Cr dissolved in water form coating is increased. This is because the organic compounds are decomposed by heat and the inorganic materials the bonds of which with the organic materials have been cut are dissolved in water. The chromium compounds the bonds of which with the organic materials have been out show water-solubility but when the temperature is further increased, Cr+ is converted into Cr+ which results in forming watersoluble coating. Such coating shows heat resistance as well as layer-boundary resistance even after annealing for relieving strains.

The same behavior as in the above example was shown, when replacing adipic acid by any other compound belonging to saturated dicarboxylic acids having at least 4 carbon atoms such as succinic acid, glutaric acid and pimelic acid. In general, if the addition amount of such organic compounds is small, lustrous blackbrown coating is obtained and if the addition amount is large, greenish transparent coating is obtained.

By using BdO, SrO, BaO, CuO and CdO instead of using MgO or CaO, almost same results as in the examples can be also obtained. In addition the use of the hydroxides of these metals or alkaline earth metals caused, of course, the same results but by using NaOH or KOH, water-insoluble coating cannot be formed. These results show that the ions in the solutions, such as, Be+ Mg+ Ca, Sr, Ba, Cu+ Zn, Cd, Al+ etc., contribute to the stability of coating but the ions, such as, Na+ and K+ do not contribute to the stability of coating.

Even when using an aqueous solution containing only a water-soluble Cr+ compound such as chromium trioxide and the water-soluble saturated dicarboxylic acid having at least 4 carbon atoms, coating can be obtained, but indicating a peculiar winding curve as shown in FIG. 1, which is considered to be caused by the action of iron ions formed from the electric iron sheet to the formation of coating. When the pH of the aqueous solution containing only chromium trioxide and the watersoluble dicarboxylic acid having at least 4 carbon atoms is kept to about 5-6 by adding NaOH or KOH into the solution, water-insoluble coating can not be obtained. This is because Na+ ion or K ion does not contribute to the stability of coating and also the surface of the electric iron sheet is hardly attacked by the coating solution since the pH of the solution is increased.

The formation of coatingfrom an aqueous solution containing only MgO and CrO needs a high temperature treatment, which is substantially different from the process of this invention, in which the low-temperature treatment is one of the main features.

Moreover, in the process of this invention, it is unnecessary to carry out the heating processin a reducing atmosphere. Further, the compounding ratio of CrO to the bivalent metal ions as of Mg, Cd, Be, Ba, Sr, Ca, Cu, etc. in this invention is not restricted to a definite one. For example, if the mixing ratio of MgO and CrO is 1:2 mole ratio or 1:4 mole ratio, the same curve as in FIG. 1 can be obtained.

The reaction rate of the aqueous solution of chromium trioxide and the organic compound is influenced generally by the pH value of the solution, and the oxidation re action of the organic compound proceeds quickly and precipitates are easily formed as the pH is decreased. When, for example, an alkaline earth metal oxide is added into the aqueous solution of chromium trioxide, the pH of the solution is increased, the oxidation reaction of the organic compounds in the aqueous solution be comes slow, the formation of precipitates becomes difiicult. For example, in the case of Example 2, the pH of the solution is 6.5 and the solution is very stable without forming precipitates but in this case, if MgO is not added in the system, the pH is 0.4 and precipitates are easily formed, which makes the continuous coating procedure diflicult. In general, in the case of using a carboxylic acid as the organic compound, the prepared solution is stable without influence with the pH but in the case of using the alcohols or sugar, the solution is unstable if the pH value is low, hence it is preferable in stability of solution to carry out the coating at a pH higher than 3. However, it should be understood that the coating process of this invention can be conducted even at the pH lower than 3.

The coating formed by the coating solution in Example 1 shows lustrous yellow brown. As the results of experiments, it was confirmed that the coating was not infected by a transformer oil, trichloroethylene, and Freon gas. The coating obtained in Example 1 had a good adhesive property, that is,when the iron sheet was bent to an angle of 180 along a rod of 10 mm. in diameter, no stripping phenomena occurred. The coating had an excellent rust resistance.

The results of measuring the layer-boundary resistance by Method 2 in ASTM are as follows:

Ohm-sq. cm./ sheet No. 1 No. 2

The layer-boundary resistances of coatings after subjection to annealing for relieving strain for 4 hours at 800 C. in an atmosphere of hydrogen 10% and nitrogen 90% are as follows, which are superior to conventional phosphate-containing coatings:

Ohm-sq. cm./sheet 85.7

The space factor of the electric iron sheets before and after the formation of coatings are as follows:

Before After N o. 1 97. 3 96. 8 No. 2 97. 06. 5

Max. shearing load, kg.

Iron

By the phosphate invention coating No. 1 8, 900 3, 500 N o. 2 4, 000 2, 500

Having thus described the invention, what is claimed is:

1. A process for the formation of an electric insulating coating on an electric iron sheet, comprising the steps of applying on the surface of an electric iron sheet a reducing solution consisting essentially of 05-20% of hexavalent chromium ion and 0.38% of at least a bivalent metal ion selected from the group consisting of copper ion, cadium ion, and alkaline earth metal ion, said reducing solution having been further added with 0.18% of a water-soluble saturated aliphatic dicarboxylic acid reducing agent having at least 4 carbon atoms and having a miscibility of above 15 minutes with at least one of these ions at room temperature, and heating it for 10-180 seconds to ISO-300 C.

2. A process for the formation of an electric insulating coating on an electric iron sheet, comprising the steps of applying on the surface of an electric iron sheet an aqueous solution consisting essentially of 05-20% of hexavalent chromium ion, 1-5% of at least one of hivalent alkaline earth metal ions, and 01-20% of phosphoric acid ion, said aqueous solution having been further added with 0.l8% of water-soluble saturated aliphatic dicarboxylic acid reducing agent having at least 4 carbon atoms and having a miscibility of above 15 minutes with at least one of these ions at room temperature, and heating it for 10-180 seconds to ISO-500 C.

3. A process for the formation of an electric insulating coating on an electric iron sheet, comprising the steps of applying on the surface of an electric iron sheet an aqueous solution consisting essentially of 25-10% of hexavalent chromium ion and l-5% of at least a member selected from the group consisting of copper ion, calcium ion, magnesium ion and barium ion, said aqueous solution having been further added with 0.18% of at least a member selected from the group consisting of succinic acid, glutaric acid, adipic acid and pimelic acid and heating it for 15-30 seconds at 250-280" C.

4. A process for the formation of an electric insulating coating on an electric iron sheet, comprising the steps of applying an aqueous solution consisting essentially of 25-10% of hexavalent chromium ion and 1-5% of at least a member selected from the group consisting of copper ion, calcium ion, magnesium ion and barium ion, said aqueous solution having been further added with 0.1-8% of at least a member selected from the group consisting of succinic acid, glutaric acid, adipic acid and pimelic acid and 0.0l-0.5% of a nonionic surface active agent of polyethylene glycol series, and 1-8% of boric acid and heating it for 10-180 seconds to temperatures of -300 C.

5. A process for the formation of an electric insulating coating on an electric iron sheet, comprising the steps of applying on the surface of the electric iron sheet an aqueous solution consisting essentially of 25-10% of hexavalent chromium ion and 1-5 of at least a member selected from the group consisting of copper ion, calcium ion, magnesium ion and barium ion, said aqueous solution having been further added with O l-8% of at least a member selected from the group consisting of succinic acid, glutaric acid, adipic acid, and pimelic acid and 0.1% of a nonionic surface active agent of polyethylene glycol series, and 25% of boric acid, and 0.1-20.0% of phosphoric acid, and heating it for 10-180 seconds in a temperature range of ISO-500 C.

References Cited UNITED STATES PATENTS 2,472,592 6/ 1949 Kiefer 1486.2 2,484,242 1 0/1949 Nagel et al. 1486.16 2,516,685 7/1950 Douty et a1. 1486.16 X 2,753,282 7/1956 Perry 1486.16 2,768,104 10/ 1956 Schuster et 211.

2,835,618 5/1958 Keller et al. 148-6.16 2,901,385 8/1959 Curtin 1486.16 2,909,454 10/1959 Neish l48-6.2 3,207,636 9/1965 Wada et a1. 1486.16

RALPH J. KENDALL, Primary Examiner U.S. Cl. X.R. 148-113 

